Gas plasma basically consists of highly excited gas/vapor species (i.e., molecules, ions, free radicals) produced by bombarding gases with energy. Some familiar examples of gas plasma are sun (thermal plasma) and neon signs (cold plasma). The temperature of the constituents in the thermal plasma is in the thousands of degrees, whereas in the cold plasma, the temperature is from ambient to a few hundred degrees. Corona discharges lie in between these two plasmas and may be termed hybrid plasmas.
The use of cold plasma for the "surface modification" of polymers has increased significantly during the past 10-15 years. The desired surface modifications have included increased wettability.sup.1-4, hydrophilicity.sup.5 and bonding.sup.1,2,4,6. Other areas have included the grafting of monomers onto polymer surfaces.sup.7-10 and the deposition of thin protective coatings of organic and inorganic precursors on metal.sup.11-13 and polymer surfaces.sup.14. A number of papers on the surface modification of polymers by corona discharge treatment have also appeared.sup.15-19.
Cold plasma treatment for surface modification can be divided into the following classes according to the type of vapor and procedure employed.sup.6 :
I. Chemically inert plasma using Ar, He, etc. PA0 II. Nonpolymerizable reactive-gas plasmas using N.sub.2, NH.sub.3, air etc. PA0 III. "Grafting" plasmas in which the substrate is first activated in an Ar plasma and then immediately exposed to a polymerizable vapor in the absence of plasma. PA0 IV. "Polymerizable" reactive gas plasmas which can employ both unsaturated and saturated hydrocarbons, and organometallic molecules. The resulting polymer coatings in such cases are usually significantly different from the precursors both in molecular structure and composition. This is because the "polymerization" process in this case involves considerable fragmentation of the precursors and recombination of some of the fragments. Ablation of the deposited coating can also be important in this case. PA0 (1) Whitby et al, "Synthetic Rubber," John Wiley & Sons, Inc., New York, 1954; PA0 (2) Schildknecht, "Vinyl and Related Polymers," John Wiley & Sons, Inc., New York, 1952; PA0 (3) "Encyclopedia of Polymer Science and Technology," Interscience Publishers a division of John Wiley & Sons, Inc., New York, Vol. 2 (1965), Vol. 3 (1965), Vol. 5 (1966), Vol. 7 (1967) and Vol. 9 (1968) and PA0 (4) Bovey et al, "Emulsion Polymerization," Interscience Publishers, Inc., New York, 1955.
Needless to say, the first two of the above are the simplest of the cold plasma surface treatments. While limited in scope, they have been shown to result in significant adhesion improvement of the treated substrates. It was found that treatment of poly(monochloro-p-xylene) in Ar, He and O.sub.2 plasma increased its adhesion to urethane coatings by a factor of ten over the untreated control.sup.4. Power levels used were 50-100 watts and treatment times ranged from 1-6 minutes. Water contact angles were found to decrease in all cases, but in the case of O.sub.2 plasma, oxygen was found to be incorporated into the surface polymer. Interestingly, Ar and He treatments gave somewhat higher adhesions than the oxygen treatment. The beneficial effects of plasma treatment were found to persist in at least up to one week of storage before application of a urethane coating.
Treatment of non-adhesive activated polyester (PET) filaments and cords in various gas plasmas (N.sub.2, Ar, He, H.sub.2, NH.sub.3, H.sub.2 O, CO.sub.2, O.sub.2) was found to increase cord-RFL rubber H-adhesion by about 100% over the untreated control.sup.2. Plasma treatment of PET, therefore, could replace a primer treatment in a two-step polyester dip system. All of the above gases gave essentially equivalent adhesion improvement. Power levels used were 75-275 watts and treatment times ranged from 4-34 seconds. Gas pressures were between 0.5-1.5 torr. A number of patents.sup.20,21 claiming the use of plasma treatment for adhesion improvement in polyester cord-RFL-rubber system have issued.
It has been proposed that the increase in adhesion of polyester to rubber after plasma treatment resulted from crosslinking of the surface region inducing increased cohesive strength near the interface.sup.23,24. It, also, has been proposed that plasma treatment might have three effects: (1) increase in the potential for strong bonding by introducing polar groups, free radicals, etc., at the polymer surface; (2) increase in surface crosslinking and the contaminant energy dissipation; and (3) increase in elastic modulus through crosslinking.sup.25.
Plasma treatment of KEVLAR for adhesion to epoxy resin gave up to 120% improvement in the peel strength adhesion of KEVLAR-epoxy laminates as a result of the plasma treatment.sup.6,22. All four classes of plasma described above were used. Generally Ar, N.sub.2, NH.sub.3 and air plasmas gave improvements of 30-80% in these studies. No adhesion studies for KEVLAR-rubber systems were carried out by these authors.